The United States Department of Energy's Pittsburgh Energy Technology Center has proposed a program entitled "Engineering Development of Coal-Fired High Performance Power Generation System" (DOE PRDA No. DE-RA22-90PC90159). In this system a combined Brayton-Rankine cycle is used to generate electricity. FIG. 1 of this patent application discloses a schematic of the DOE gas turbine cycle. In that cycle disclosed in FIG. 1, a high temperature furnace is required to generate steam and air to drive the combined Brayton-Rankine cycle. This invention includes a furnace which can be used in the cycle but was conceived and developed without DOE funding and the United States government acquires no rights in/or to this invention. However the cycle is background to this invention.
With respect to coal-fired boilers, it is known to position a plurality of coal-fired burners in a wall so that the burners develop a two dimensional array or matrix of flame fronts which impinge upon a plurality of heat exchanger tubes extending through the boiler. Carbon and/or ash from the coal eventually coat the heat exchanger tubes making them less effective and materially shortening their life. That is, not only does the coating interfere with heat transfer to the tube, but the coating chemically reacts with the tube to cause disintegration of the tube. In addition, it is known that the maximum tensile and ultimate stresses of alloy tubes are significantly reduced when temperature increases from 1100.degree.-1200.degree. F. to 1600.degree.-1800.degree. F. The stress reduction at elevated temperature becomes further aggravated when ash coats the tube, thus rendering conventional alloy heat exchange tubes unsuitable for high temperature applications in sooty, coal combustion atmospheres. To some extent the adverse effects of the coating are reduced by periodically purging high velocity gas or air flow followed by boiler cleaning of loose carbon and/or ash particles. While purging may alleviate the problem in conventional low temperature boiler applications, in high temperature application, the carbon or ash coats or fuses itself to the heat exchanger tubes and cannot be dissipated by the purge cycles.
In addition, prior art, coal-fired boilers do not operate at the temperatures discussed herein and produce NO.sub.x during combustion at emission levels far surpassing proposed and now existing NO.sub.x emission levels. Such emission levels have required conversion of coal-fired burners to natural gas or other forms of energy. With respect to NO.sub.x emissions from coal-fired burners per se, research work on staged combustion with pulverized coal burners conducted by the International Flame Research Foundation has demonstrated that pulverized coal burners with staged combustion can produce low NO.sub.x emissions and that such burners could be retrofitted to water-tube boilers. That is, it is known to use the staged combustion approach to limit the upper flame temperature of the coal fired burner to keep NO.sub.x emissions low. However, the staged combustion approaches typically used in the prior art either are ineffective to limit the temperatures to the desired ranges or produce localized hot spots or temperature spikes whereat NO.sub.x compounds form.
The prior art clustered burners used in boilers blends or molds the burner flames together into one large flame mass which limits the ability of such arrangement to effect uniform heat transfer by radiation. At high temperatures, it is known that heat transfer principally occurs by radiation. The cluster prior art boilers cannot and do not present a "transparent" flame. The massive flame front serves as a radiation front driving temperatures to excessively high levels at certain areas of the heat exchange tubes which "see" the flame front This not only distorts heat transfer uniformity and eventually thermally destroys the tubes but significantly contributes to high NO.sub.x formation levels.